1. Field of the Invention
The present invention relates to a control system for controlling a continuously variable transmission which, even during the warm-up operation of an engine including an engine cool condition in which the amount of the air allowed to pass by an idle speed control valve increases, in deceleration due to the full-closed condition of a throttle valve, can secure intake pipe negative pressure downstream of the throttle valve, which can be used as the power source of a master back defining a brake assist system, to thereby be able to obtain a proper brake force.
2. Description of the Related Art
Conventionally, in a control system for controlling a continuously variable transmission, a target primary pulley revolution number is set by referring to a basic change gear characteristic map which gives a target primary revolution number with the opening angle of a throttle valve (throttle opening angle) and a vehicle speed as parameters, a target change gear ratio is calculated from a ratio between the thus set target primary pulley revolution number and an actual secondary pulley revolution number, and an actual change gear ratio, which is given by a primary pulley capable of inputting therein the revolution of an engine and a secondary pulley capable of inputting therein power from the primary pulley through power transmission means, is so controlled as to converge to the target change gear ratio, whereby the change gear ratio is followingly controlled so that the actual primary pulley revolution number converges to the target primary pulley revolution number.
Due to this, the change gear ratio can be set in a continuously variable manner from low up to overdrive in accordance with the basic change gear characteristic map, and thus the change gear ratio can be properly set according to the driving conditions.
In the basic change gear characteristic map, for example, as shown in FIG. 11, with a horizontal axis expressing a vehicle speed and a vertical axis expressing a target primary pulley revolution number NP, there is formed a change gear chart which is enclosed by a low side change gear line, an overdrive (OD) side change gear line, and throttle full-open and full-closed change gear lines, while the target primary pulley revolution number can be set according to the vehicle speed and throttle opening angle. For example, when deceleration is carried out by closing the throttle completely from a high vehicle speed, the change gear ratio is controlled in such a manner that the primary pulley revolution number passes through the OD side change gear line and throttle full-closed change gear line.
Here, between the engine and the continuously variable transmission, there is interposed an electromagnetic clutch or a torque converter. Therefore, when the continuously variable transmission and engine are directly connected together during the running condition of the vehicle, the revolution number of the engine coincides with the primary pulley revolution number. And, in deceleration due to the full-closed condition of the throttle valve, because of a brake force given from the driving wheel side, the engine revolution number depends on the revolution number of the primary pulley of the continuously variable transmission connected to the engine.
By the way, in recent years, for improvement in fuel consumption and vehicle running performance, there is found a tendency that the throttle full-closed change gear line is lowered down to the low revolution number side.
However, in an engine cool condition in which the temperature of an engine coolant is low, the engine is held in an unstable condition. Therefore, in deceleration due to the full-closed condition of the throttle valve, if the throttle full-closed change gear line is lowered to thereby decrease the primary pulley revolution number so that the revolution number of the engine connected to the primary pulley is reduced, then there can be generated uncomfortable vibrations or engine stall because of the engine friction depending on the viscosity of oil, deteriorated combustion efficiency of the engine and the like.
In order to cope with the above troubles, in Japanese Patent Unexamined Publication No. 58-180865 of Showa, there is disclosed a technology in which, only in the engine cool condition where the engine coolant temperature is lower than a set value, a throttle valve opening angle, which is used as a parameter when setting a target primary pulley revolution number, is uniquely raised by a given opening angle to thereby shift the whole of the basic change gear characteristic to the high revolution number side. Accordingly, the primary pulley revolution number is increased and thus the engine revolution number is increased to thereby improve the combustion of the engine, whereby the above-mentioned uncomfortable vibrations or engine stall can be prevented.
On the other hand, recently, in most of vehicles including a mini-sized motor vehicle, there is mounted a master vac (brake power assist) as a brake assist system. This master vac, as known well, is composed of a brake booster of a diaphragm type. In the master vac, as the power source thereof, there is used intake pipe negative pressure which is produced downstream of the throttle valve of the engine. That is, the intake pipe negative pressure is used to assist a brake pedal effort acting on an operating rod so disposed as to adjoin a brake pedal, so that large master cylinder operation pressure can be generated with a small brake pedal effort. Such assisting use of the intake pipe negative pressure can provide a brake force several times larger than that obtained when only the brake pedal effort is used.
Here, generally, the intake pipe negative pressure, which is generated downstream of the engine throttle valve and is used as the power source of the master back, is substantially determined by a difference between the intake amount demanded by engine (.apprxeq.displacement.times. engine revolution number) and the intake amount supplied to engine (.apprxeq.sum of throttle valve passing air flow amount and idle speed control valve passing air flow amount).
Therefore, if the throttle full-closed change gear (transmission) line is lowered to the low revolution number side in the above-mentioned manner, then the engine revolution number in deceleration due to the full-closed condition of the throttle valve is reduced to thereby decrease the engine demand intake amount, which makes it difficult for the intake pipe negative pressure to be generated. Also, during the engine warm-up operation on the way to the completely warmed condition of the engine including an engine cool condition, in order to compensate the engine friction due to the viscosity of oil and worsened engine combustion performance, the opening angle of an idle speed control valve (which is hereinafter referred to as ISC valve) disposed in a bypass passage bypassing the throttle valve is set in such a manner as to increase according to the engine coolant temperature. Thus, the ISC valve passing air flow amount can be increased and the amount of the air to be supplied to the engine (that is, engine supply air amount) can be thereby increased.
Therefore, in deceleration during the engine warm-up operation on the way to the engine completely warmed condition including an engine cool condition, even if the throttle valve is closed full, the engine supply air flow amount increases due to an increase in the ISC valve passing air flow amount, which makes it further difficult to generate the intake pipe negative pressure downstream of the throttle valve. Further, even in a condition where the warm-up of the engine is completed (in a completely warmed condition), during the operation of an auxiliary machine such as an air conditioner or the like, the opening angle of the ISC valve is caused to increase in order that, in correspondence to an increase in the engine load due to the operation of a compressor for the air conditioner in operation, the ISC valve passing air flow amount can be increased and the engine supply air flow amount can be thereby increased. This, however, makes it difficult for the intake pipe negative pressure to be generated.
Here, when the brakes are in operation, normally, in a state where the press-down of an accelerator pedal is removed and the throttle valve is closed fully, the brake pedal is pressed down. For this reason, especially, in deceleration due to the full-closed condition of the throttle valve during the engine warm-up operation on the way to the engine completely warmed condition including an engine cool condition, the intake pipe negative pressure downstream of the throttle valve runs short and thus the assist force by the master back is reduced, with the result that the holding of the brakes is worsened. The smaller the displacement of an engine to be carried on board a vehicle is, the more outstanding this tendency is.
By the way, in deceleration, because the throttle valve is fully closed, the engine supply air flow is provided by the idle speed control valve which is passing air flow amount and, therefore, in this state, it is possible that the opening angle of the ISC valve is forcibly decreased to thereby earn the intake pipe negative pressure downstream of the throttle valve. However, the ISC valve passing air flow amount depending on the opening angle of the ISC valve is set in such a manner as to compensate the engine friction, the drive torque of auxiliary devices and the like. Therefore, in deceleration due to the full-closed condition of the throttle valve, if the ISC valve is forcibly closed to thereby decrease the ISC valve passing air flow amount, then there can be induced, in sudden deceleration, an engine stall due to the shortage of the air amount supplied to the engine. That is, the forced reduction of the opening angle of the ISC valve is not be able to solve the problem found in the conventional structure fundamentally.
Also, in the above-cited conventional structure, only in the engine cool condition where the engine coolant temperature is lower than a set value, a throttle valve opening angle, which is used as a parameter when setting a target primary pulley revolution number, is uniquely raised by a given opening angle to thereby shift the whole of the basic change gear characteristic to the high revolution number side, whereby the primary pulley revolution number can be increased and the engine revolution number can be thereby increased. Therefore, in deceleration due to the full-closed condition of the throttle valve, when the engine coolant temperature is lower than a set value, due to the increased engine revolution number, the intake pipe negative pressure downstream of the throttle valve can be secured to thereby enhance the assist force of the master back which uses the intake pipe negative pressure as its power source. However, in an area where the engine coolant temperature is equal to and higher than the set value, the basic change gear characteristic is not shifted but remains as it is. That is, the brake assist force by the master back varies extremely with the set value as the border thereof, which raises an inconvenience that the driving feeling of the vehicle can be worsened.
Further, in the above-cited conventional structure, there is employed a technique which, in order to prevent the above-mentioned uncomfortable vibrations and engine stall, adds the uniquely defined given opening angle to the throttle valve opening angle. However, when the ISC valve passing air flow amount varies continuously as the ISC valve opening angle varies according to the engine coolant temperatures, such conventional technique is able to cope with differences between the continuously varying ISC valve passing air valve flow amounts only in a certain special range thereof; that is, such technique is not able to match the brake assist force of the master back to the differences between the continuously varying ISC valve passing air flow amounts over the whole range thereof. Also, when the engine coolant temperature is lower than the set value, the whole basic change gear characteristic is uniformly shifted by a given throttle valve opening angle and, therefore, due to differences between the engine coolant temperatures with the set value as the boundary thereof, the basic change gear characteristic of the continuously variable transmission is caused to vary as a whole, which raises an inconvenience that the driving feeling can be further worsened and thus controllability on the continuously variable transmission can also be worsened.